48 GEOLOGICAL SURVEY OF TH LERRIT°OVIES. 
Genus BYTHINELLA Moguin-Ta don. 
BYTHINELLA GREGARIA M :k. 
Plate 19, figs. 6a and 60. 
Bythinella gregaria Meek, 1871, An. Rep. U. S. Geol. Sur. Terr. for 1870, p. 317. 
The following description and remarks of Mr. Meek embrace all that 
has yet been learned concerning this species: 
‘¢ Shell small, conoid-subovate; spire rather elevated; volutions five, 
rounded or very convex; suture strongly impressed; aperture ovate or 
slightly longer than wide, with the upper extremity subangular, and the 
lower rounded; inner lip not reflected, and leaving by its side a very 
small umbilical impression that seems not to perforate the axis. Surface 
smooth, or only showing, under a strong’ magnifier, very minute lines of 
growth. 
“ Length, 0.15 inch; breadth, 0.08 inch ; length of aperture, 0.06 inch; 
breadth of aperture scarcely 0.04 inch. 
“This little shell so nearly resembles in form and proportions the fig- 
ures of Bythinella tenuipes of Couper, that it is with some hesitation 
I have concluded to regard it as a distinet species. As that shell, how- 
ever, is described as having its suture ‘slightly impressed,’ and as being 
‘subumbilicated’ while that under consideration has its suture very 
deep, and could not be properly described as even swbumbilicated, I do 
not feel warranted in referring our Tertiary form to the existing species. 
‘‘Of course we have no certain means of determining whether we ought 
not to call this shell Amnicola gregaria, or Pomatiopsis gregaria, instead 
of referring it to Bythinella, the distinction between these two groups 
being mainly based on characters not apparent in the shell. The fact, 
however, that it is found in vast numbers associated with a small Plan- 
orbis and millions of the carapace-valves of a minute Cypris, would seem 
to indicate that it was aquatic in its habits, like Bythinella and Amnicola. 
It is true terrestrial shells are often swept by_streams into lakes, and 
deposited along with those of aquatic species; but it is exceedingly im- 
probable that millions cf so small a shell as this would have been de- 
posited all together, so as almost to form an entire bed of limestone, es- 
pecially without some other terrestrial types. 3 
“6 Locality and position.—Pacifie Springs [Wyoming], Tertiary.” 
